1. Field of the Invention
The invention relates to improvements and modifications of an osmotically driven fluid dispenser.
2. Description of the Prior Art
The invention is an improvement in the osmotically driven dispensers described in commonly owned U.S. Pat. No. 3,987,790, U.S. Pat. No. 3,995,631 and U.S. Pat. No. 4,034,756. Therefore, these patents are believed to be relevant prior art, particularly the mini-osmotic pump described in these patents. The mini-osmotic pump's components are an inner flexible bag that holds the drug charge, an intermediate layer of an osmotically effective solute composition, such as an inorganic salt, that encapsulates the bag, an outer shape-retaining membrane that is at least in part permeable to water and that encapsulates both the layer of osmotically effective solute composition and the bag, a plug that seals the open end of the bag, and a filling/discharge port that communicates with the interior of the bag.
In operation the bag is filled with drug solution via the filling/discharge port and placed in an aqueous environment, such as a body cavity or within body tissue. Water is imbibed from the environment by the osmotically effective solute through the membrane into the space between the inner flexible bag and the membrane. Since the bag is flexible and the membrane is rigid, the imbibed water squeezes the bag inwardly, thereby displacing drug out the filling/discharge port.
Mini-osmotic pumps of the above described structure and operation perform well but are not beyond being improved. Two shortcomings of the mini-osmotic pump are: (1) the layer of osmotically effective solute is applied by dipping the inner bag into a suspension of an osmotically active solute, which is a highly labor intensive process, and (2) uniformity and reliability of the thickness of the solute layer are difficult to assure in large scale production due to the inherent variability of the dipping process. The first shortcoming affects the ease and cost of production. The second shortcoming affects the average release rate of drug, the constancy of the release rate, and the duration of release. The present invention is directed toward eliminating or reducing both of these shortcomings.